Associations between breast cancer and exposure to environmental carcinogens are often weak or inconsistent in the epidemiologic literature, even when strong experimental data exists. The strongest support for environmental carcinogenesis comes from mouse models showing that exposures such as ionizing radiation (IR) and polycyclic aromatic hydrocarbons (PAHs) increase mammary tumor formation. These same experimental models have shown that parity has a p53-mediated, protective effect against carcinogenesis. This application proposes a comparative toxicology approach for dissecting the biological pathways that lead to environmental carcinogenesis of the breast, with emphasis on interactions between environmental carcinogens, the p53 pathway, and parity. Three classes of environmental exposure, each with a strong epidemiologic and toxicological literature, have been selected for study: IR, PAHs, and organochlorines (OCs). The first two classes (IR and PAHs) both cause DNA damage and induce p53-dependent DNA damage responses, but the DNA damage mechanisms are different. In contrast, OCs do not act by directly damaging DNA, and thus, provide a negative control for p53-dependent signaling. OCs also provide a comparison for the PAHs as both PAHs and OCs can activate the aryl hydrocarbon receptor (AhR) and perturb endocrine action in the breast. In Aim 1, we will assess the p53 dependent effects of each toxicant using isogenic human breast cell lines (with and without expression of p53RNAi to knockdown p53 expression) and nulliparous Trp53+/+ and Trp53-/- mice. In Aim 2, we will examine gene expression profiles associated with parity in humans (using reduction mammoplasty samples) and mice, with specific attention to identifying p53-regulated gene expression. In Aim 3, the interaction between parity and environmental carcinogenesis will be assessed by comparing transcriptional responses to toxicants in parous and age-matched nulliparous mice. Differential responses to toxicants will be verified in explant cultures of human breast tissue from nulliparous and parous women (excess tissue from reduction mammoplasty). Thus, each aim will integrate human and mouse responses to define responses to toxicants and parity. The data from all three aims will be integrated to allow a complete, multifactorial evaluation of how environmental exposures to these toxicants, p53 signaling, and parity status interact to affect breast signaling, and ultimately carcinogenesis. These studies will define the mechanisms of environmentally-induced breast cancer, anchored on genotype-phenotype correlations that define susceptibility, and will identify new candidate biomarkers of susceptibility that can be used in future animal and human studies.